1. Field of the Invention
The invention relates to transport refrigeration systems, such as the refrigeration systems used for conditioning perishable and frozen loads carried by trucks, trailers and containers.
2. Description of the Prior Art
Transport refrigeration systems commonly employ an internal combustion engine as the prime mover for the refrigeration compressor. It is also common to operate the engine in one of two selectable speeds according to the requirements of the load being conditioned. For example, when the temperature of the served space is being initially pulled down towards set point, the refrigeration system would operate in a high speed cooling mode until the temperature of the served space reaches a predetermined temperature relative to set point. The engine would then be switched to low speed cool until the set point is reached. Below set point, the system would operate in a low speed heating mode, and it may operated in a high speed heating mode if the temperature drops below a predetermined value relative to set point. U.S. Pat. Nos. 4,325,224 and 4,419,866, which are assigned to the same assignee as the present application, set forth transport refrigeration systems which utilize different heating and cooling modes, including the high and low speed modes described for both heating and cooling.
Transport refrigeration systems additionally commonly employ an electrical solenoid for operating the throttle of the internal combustion engine when the system requires a change from one compressor speed to the other. In some systems the prime mover also provides the motive force for driving evaporator and condenser fans and/or blowers, hereinafter called "air mover means", via a suitable drive arrangement which has a selectable drive ratio such that the air mover means may be driven at substantially the same speed at both speeds of the prime mover. These systems automatically change the drive ratio when the speed relay picks up, and when the speed relay drops out. If the throttle solenoid should fail, either mechanically or electrically in the high speed mode, the change in drive ratio would still occur and overspeeding of the air mover means would result.
It would be desirable to be able to drive the air mover means from the prime mover without the possiblilty of a failure which could cause overspeeding of the air mover means.